On the surface it looks like a really great idea and I applaud you for having the creativity and desire to see this through.

On the other hand, I see big, huge, red, flashing, neon warning lights going off in my head, thinking: danger! I would be super-careful in choosing a "caber" especially for the 6-10 set. I agree that either a cardboard tube or broomstick would be most appropriate (as opposed to a 4x4 or similar). If a child lost control of that 6-foot 4x4, he could hit himself with it trying to toss it or injure another child. It could be a lawsuit waiting to happen (worst case scenario) or at the very least, an unpleasant ride to the emergency to get some stitches.

Again, I don't want to rain on what seems to be a really good idea, but in today's litigious society, I'd make sure that everything were SAFE, and even then it might not be a bad idea to get a signed waiver from the parents.